The present invention relates to the electroless deposition of copper onto a substrate surface and, more particularly, to a method for carrying out such depositing in a manner which minimizes the build-up of non-adherent copper metal in the depositing or plating bath.
It is well-known that copper can be deposited electrolessly onto suitably catalyzed conductive and non-conductive surfaces by means of aqueous plating baths containing a soluble source of copper ions (e.g., copper salt), a reducing agent, a complexing agent, and pH adjusting agents. The action of the reducing agent on the dissolved copper ions in the presence of the suitably catalyzed (activated) substrate brings about deposition of copper metal onto the surfaces.
Typical early-vintage electroless copper depositing baths relied upon formaldehyde as the reducing agent, and are still in widespread use today notwithstanding recognized potential problems relating to toxicity of formaldehyde vapors. A significant advance in the art was achieved in U.S. Pat. No. 4,209,331 which describes formaldehyde-free electroless copper depositing solutions based upon hypophosphite reducing agents.
The formaldehyde-reduced electroless copper baths are "autocatalytic", i.e., once a copper layer is electrolessly deposited therefrom onto the activated substrate surface, the deposited layer serves to catalyze yet further deposition. This is a desirable feature of electroless plating baths since it permits the build-up of copper on substrate surfaces to substantial thicknesses dictated solely by maintenance of plating conditions and bath concentrations for predetermined periods of time.
In contrast, the hypophosphite-reduced electroless copper baths such as described in U.S. Pat. No. 4,209,331 are non-autocatalytic, i.e., once the activated substrate has been coated with a thin layer of the deposited copper, the deposition reaction stops or becomes uneconomically slow. It is known, however, that hypophosphite-reduced electroless copper baths can be modified compositionally and/or via process techniques, to achieve desirable "autocatalytic" plating. For example, in U.S. Pat. No. 4,459,184, an inherently non-autocatalytic hypophosphite-reduced electroless copper bath (such as described in U.S. Pat. No. 4,209,331) is first used to produce on the activated substrate the thin copper deposit possible with such baths. Thereafter, an electrical current of negative potential is continuously applied to the substrate workpiece to bring about additional copper deposition, essentially in the nature of an electroplating process using an electroless plating bath.
In U.S. Pat. No. 4,265,943, an otherwise non-autocatalytic hypophosphite-reduced electroless copper bath is rendered autocatalytic by inclusion in the bath of sources of non-copper ions (e.g., cobalt and/or nickel ions) which serve as autocatalysis promoters. In U.S. Pat. No. 4,671,968, it was taught that the baths of U.S. Pat. No. 4,265,943 do not in fact operate autocatalytically in many situations (e.g., in the attempted plating of non-conductive surfaces of through-holes formed in a copper-clad printed circuit board substrate), but can be made to do so by briefly applying to the workpiece an electric current of negative potential, whereafter the applied potential is terminated and the initiated electroless plating then continues autocatalytically.
By their very nature, electroless copper baths which are inherently autocatalytic, or which are operated in a way so as to render them autocatalytic in use, can cause difficulties in the plating process During the course of plating of the suitably activated substrate surfaces of the workpiece, copper metal may also (undesirably) deposit on non-workpiece areas. One means by which this occurs is by virtue of minute impurities (e.g., dirt particles) present in the bath which can serve essentially as minute "substrates" for copper metal deposited from the bath. Copper which adheres to such impurities generally will not adhere to the workpiece surface and thus tends to accumulate in the plating vessel. By reason of autocatalysis, these initially minute copper particles act as seeds or nucleation sites for still further build-up of copper thereon, to the point where, over the course of the plating process or multiple plating processes using the bath, they can grow to fairly substantial size and in fairly substantial numbers. As these particles grow in size and weight, they eventually fall to the bottom of the plating tank. These copper fines (often referred to as sanding) thus undesirably consume bath components intended for plating the workpiece, and require replenishment of the bath and the obvious cost penalty associated therewith. Still further, in many situations, the copper fines can result in clogging of devices (e.g., filters) associated with the plating vessel. As a result, periodic cleaning of the vessel may be necessary, again adding time and labor costs to the operation of a plating line.